haskell facebook exceptions - facebook

Question

After reading this I tried
{-# LANGUAGE OverloadedStrings, NoMonomorphismRestriction #-}
module Login (
fbUrl,
fbEmail
) where
-- package http://hackage.haskell.org/package/fb
import qualified Facebook as FB
import Network.HTTP.Conduit (withManager)
app :: FB.Credentials
app = FB.Credentials "localhost" "249348058430770" "..."
url :: FB.RedirectUrl
url = "http://localhost/fb"
perms :: [FB.Permission]
perms = ["user_about_me", "email"]
--fbUrl :: Monad m => FB.FacebookT FB.Auth m Text
fbUrl :: IO Text
fbUrl = withManager $ \manager -> FB.runFacebookT app manager $ FB.getUserAccessTokenStep1 url perms
--fbEmail :: Monad m => (ByteString, ByteString) -> FB.FacebookT FB.Auth m (Maybe Text)
--fbEmail :: (ByteString, ByteString) -> IO (Maybe Text)
fbEmail c = withManager $ \manager -> FB.runFacebookT app manager $ do
t <- FB.getUserAccessTokenStep2 url [c]
u <- FB.getUser "me" [] (Just t)
return $ FB.userEmail u
module Main (
main
) where
import Login
import qualified Data.ByteString.Char8 as C
import Control.Exception
main :: IO ()
main = do
let a = ("code","test")
e <- fbEmail $ (\(x,y) -> (C.pack x, C.pack y)) a
case e of
Nothing -> print "doh!"
Just e -> print e
I get haskell-facebook: FacebookException {fbeType = "invalid_code", fbeMessage = "Invalid verification code format."} instead of doh!
With e <- try (fbEmail $ (\(x,y) -> (C.pack x, C.pack y)) a) i get
Couldn't match expected type `Either
e0 (Maybe Data.Text.Internal.Text)'
with actual type `Maybe t0'
In the pattern: Nothing
In a case alternative: Nothing -> print "doh!"
In a stmt of a 'do' block:
case e of {
Nothing -> print "doh!"
Just e -> print e }
#Daniel Fischer
let a = ("code","test")
e <- try (fbEmail $ (\(x,y) -> (C.pack x, C.pack y)) a)
case e of
Left x -> print "doh!"
Right e -> print "ok"
Ambiguous type variable `e0' in the constraint:
(Exception e0) arising from a use of `try'
Probable fix: add a type signature that fixes these type variable(s)
In a stmt of a 'do' block:
e <- try (fbEmail $ (\ (x, y) -> (C.pack x, C.pack y)) a)
In the expression:
do { let a = ...;
e <- try (fbEmail $ (\ (x, y) -> (C.pack x, C.pack y)) a);
case e of {
Left x -> print "doh!"
Right e -> print "ok" } }
In an equation for `main':
main
= do { let a = ...;
e <- try (fbEmail $ (\ (x, y) -> (C.pack x, C.pack y)) a);
case e of {
Left x -> print "doh!"
Right e -> print "ok" } }
When I add a type signature fbEmail :: Monad m => (ByteString, ByteString) -> FB.FacebookT FB.Auth m (Maybe Text) I get
Could not deduce (monad-control-0.3.1.3:Control.Monad.Trans.Control.MonadBaseControl
IO m,
resourcet-0.3.2.2:Control.Monad.Trans.Resource.MonadUnsafeIO m,
Control.Monad.IO.Class.MonadIO m,
resourcet-0.3.2.2:Control.Monad.Trans.Resource.MonadThrow
(FB.FacebookT FB.Auth m))
arising from a use of `withManager'
from the context (Monad m)
bound by the type signature for
fbEmail :: Monad m =>
(ByteString, ByteString) -> FB.FacebookT FB.Auth m (Maybe Text)
at src/Login.hs:(25,1)-(28,27)
Possible fix:
add (monad-control-0.3.1.3:Control.Monad.Trans.Control.MonadBaseControl
IO m,
resourcet-0.3.2.2:Control.Monad.Trans.Resource.MonadUnsafeIO m,
Control.Monad.IO.Class.MonadIO m,
resourcet-0.3.2.2:Control.Monad.Trans.Resource.MonadThrow
(FB.FacebookT FB.Auth m)) to the context of
the type signature for
fbEmail :: Monad m =>
(ByteString, ByteString) -> FB.FacebookT FB.Auth m (Maybe Text)
or add instance declarations for
(monad-control-0.3.1.3:Control.Monad.Trans.Control.MonadBaseControl
IO m,
resourcet-0.3.2.2:Control.Monad.Trans.Resource.MonadThrow
(FB.FacebookT FB.Auth m))
In the expression: withManager
In the expression:
withManager
$ \ manager
-> FB.runFacebookT app manager
$ do { t <- FB.getUserAccessTokenStep2 url [...];
u <- FB.getUser "me" [] (Just t);
.... }
In an equation for `fbEmail':
fbEmail c
= withManager
$ \ manager
-> FB.runFacebookT app manager
$ do { t <- FB.getUserAccessTokenStep2 url ...;
.... }
When I add fbEmail :: (ByteString, ByteString) -> IO (Maybe Text) I get
Ambiguous type variable `e0' in the constraint:
(Exception e0) arising from a use of `try'
Probable fix: add a type signature that fixes these type variable(s)
In a stmt of a 'do' block:
e <- try (fbEmail $ (\ (x, y) -> (C.pack x, C.pack y)) a)
In the expression:
do { couchTest;
u <- fbUrl;
print u;
let a = ...;
.... }
In an equation for `main':
main
= do { couchTest;
u <- fbUrl;
print u;
.... }

Answer 1

try adds another layer on top of your result. To make this example simple, I'm catching all exceptions by using the catch-all SomeException.
To make it a bit more clear, here are the result types with type signatured added:
e :: Maybe Text <- fbEmail $ (\(x,y) -> (C.pack x, C.pack y)) a
Compared to this use of try with the exception type equal to SomeException:
e :: Either SomeException (Maybe Text) <- fbEmail $ (\(x,y) -> (C.pack x, C.pack y)) a
It's simple to deal with these more complex types using pattern matching as Daniel mentioned in his comments:
{-# LANGUAGE FlexibleContexts, OverloadedStrings #-}
module Main where
import Control.Exception
import qualified Data.ByteString.Char8 as C
import Data.Text
import Network.HTTP.Conduit (withManager)
import qualified Facebook as FB
app :: FB.Credentials
app = FB.Credentials "localhost" "249348058430770" "..."
url :: FB.RedirectUrl
url = "http://localhost/fb"
perms :: [FB.Permission]
perms = ["user_about_me", "email"]
fbUrl :: IO Text
fbUrl = withManager $ \manager -> FB.runFacebookT app manager $ FB.getUserAccessTokenStep1 url perms
fbEmail :: FB.Argument -> IO (Maybe Text)
fbEmail c = withManager $ \manager -> FB.runFacebookT app manager $ do
t <- FB.getUserAccessTokenStep2 url [c]
u <- FB.getUser "me" [] (Just t)
return $ FB.userEmail u
main :: IO ()
main = do
let a = ("code","test")
e <- try . fbEmail $ (\(x,y) -> (C.pack x, C.pack y)) a
case e of
Left e -> print $ "error: " ++ show (e :: SomeException)
Right Nothing -> print "doh!"
Right (Just e) -> print e